Constructive nanolithography

ABSTRACT

A patterned organic monolayer or multiplayer film self-assembled on a solid substrate, the pattern consisting in a site-defined surface chemical modification non-destructively inscribed in the organic monolayer or multiplayer by means of an electrically biased conducting scanning probe device, stamping device and/or liquid metal or metal alloy or any other device that can touch the organic monolayer or multiplayer surface and inscribe therein a chemical modification pattern upon application of an electrical bias.

FIELD OF THE INVENTION

[0001] The present invention relates to a new template-controlledself-assembly of inorganic-organic monolayers/multilayers, referredherein as constructive nanolithography. In particular, the inventionrelates to the use of organic monolayers that are nondestructivelypatterned by various scanning probe and/or stamping devices able totouch the monolayer surface and inscribe thereon a chemical modificationpattern upon application of an electrical bias.

[0002] Abbreviations: AFM: atomic force microscope; NTS:18-nonadecenyltrichlorosilane; OTS: n-octadecyltriclilorosilane; SE:silver enhancer; STM: scanning tunnelling microscope; TFRSM:thiol-top-functionalized silane monolayer.

BACKGROUND OF THE INVENTION

[0003] The production of nanostructures and of organized organicmonolayers and multilayers as well as of semiconductor and metal quantumparticles, has resulted in some achievements in recent years. The keyissue for practical applications resides in good control of thedimensions, spatial location and stability of the assemblednanostructure. Recently, rather good control of the size of metals andsemiconductor quantum particles as well as the number and stacking orderof definite monolayers in a synthetic multilayer assembly have beenachieved. However, genuine submicrometer architecture based on a plannedassembly of nanoelements, can be achieved only by appropriate methods ofprecise positioning, spatial fixation and lateral interconnection of thedesired nanostructures. The direct chemical synthesis of nanostructureson a patterned solid template capable of defining the position andlateral dimensions of growing objects might be the proper approach fornanofabrication, provided suitable versatile templates for growing thedesired structures can be conveniently manufactured.

[0004] Surface nanopatterning of organic monolayers seems promising forpreparing such templates with the desired chemical properties on thesurface. Indeed, such patterning using conventional techniques ofoptical methods and lithographic schemes based on local degradation ofthe monolayer coating have been employed.

[0005] The possibility of achieving non-destructive surface patterningof a vinyl-terminated silane monolayer self-assembled on silicon, by theapplication of an electrical bias to a conductng atomic force microscope(AFM) tip operated in normal ambient conditions has been recentlyreported by the present inventors (Maoz et al., 1999). The tip-inducedtransformation was shown to proceed by local electrochemical oxidationof the top vinyl functions of the monolayer, with full preservation ofits overall molecular order and structural integrity. It was furthershown that such nanoelectrochemically patterned monolayers may beemployed as extremely robust, stable templates for the controlledself-assembly of organic bilayer structures with predefined size, shapeand surface location (Maoz et al., 1999).

[0006] However, post-patterning by chemical methods is subject toconstraints posed by the need of site specificity. Only the tipinscribed sites should be affected, leaving the rest of the surfaceunmodified. Therefore, the use of base monolayer that are chemicallysensitive restricts further chemical post-patterning.

SUMMARY OF THE INVENTION

[0007] It has now been found, according to the present invention, thatthe template-controlled self-assembly strategy, referred to herein as“constructive nanolithography”, can be extended to the non-destructivepatterning of monolayers with top functions different from vinyl,including also inert methyl groups, as well as to planned constructionof hybrid inorganic (e.g. metal)—organic surface nanostructures.

[0008] It has further been found according to the present invention,that organic monolayers can be patterned by various scanning probedevices including, but not being limited to, AFM, STM, combined AFM-STM,or any other device that can touch the monolayer surface and inscribe onit a chemical modification pattern upon application of an electricalbias.

[0009] The present invention thus relates, in one aspect to a patternedorganic monolayer or multilayer film self-assembled on a solidsubstrate, the pattern consisting in a site-defined surface chemicalmodification non-destructively inscribed in the organic monolayer ormultilayer by means of an electrically biased conducting scanning probedevice, stamping device and/or liquid metal or metal alloy or any otherdevice tbat can touch the organic monolayer or multilayer surface andinscribe therein a chemical modification pattern upon application of anelectrical bias.

[0010] In another aspect, the invention relates to a nanostructurecomposed of a material selected from a metal, a metal compound, siliconor a silicon compound, combined with nanoelectrochemically patternedorganic monolayer or multilayer templates self-assembled on a solidsubstrate, wherein said metal, metal compound, silicon or siliconcompound is on the top and/or in-between said organic layer templates.

[0011] The invention further relates to methods for the preparation ofsaid patterned organic monolayer or multilayer film self-assembled on asolid substrate, and of said nanostructures, and to possibleapplications thereof.

DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 depicts a schematic representation of constructivenanolithography as a generic approach to the planned surfaceself-assembly of diverse inorganic-organic nanostructures, including theself-assembly of organic, metal, and semiconductor nanoentities on topof a base organic monolayer template. Six nanofabrication routes areindicated, starting with an “inert” silane monolayer (OTS/silicon) thatis non-destructively patterned by an electrically biased AFM tip, tolocally form electrooxidized OTS (OTSeo), followed by the selectiveself-assembly of a vinyl-terminated silane overlayer (NTS) at the OTSeopolar sites defined by the tip. Subsequently, the terminal ethylenicfunctions of NTS are photoreacted with H₂S in the gas phase orchemically oxidized with aqueous (KMnO₄-KlO₄) to give the correspondingTFSM/OTSeo or NTSox/OTSeo bilayer template. Site-defined surfaceself-assembly of metallic silver, cadmium sulfide, or a third organicmonolayer can, finally, be achieved using various template-controlledprocesses. The possibilities depicted here are: (1.4) binding of Ag⁺ions to the thiol or the carboxylic acid surface functions of the TFSM(1) and respectively NTSox (4) template, followed by reduction withaqueous NaBH₄ or gaseous N₂H₄, and further development, if desired, ofthe silver nanoparticles so obtained with a silver enhancer (SE)solution; (2) binding of a gold species to the TFSM surface, followed bygold-catalyzed silver metal deposition from the SE solution; (3, 5)binding of Cd²⁺ ions to the TFSM (3) or the NTSox (5) surface, followedby the formation of CdS upon exposure to H₂S; (6) exposure of NTSox to asolution of a self-assembling silane, which results in the formation ofan organic trilayer at the tip-inscribed sites.

[0013]FIG. 2 shows simultaneously recorded friction and topographycontact mode AFM images demonstrating three distinct modes ofnanoelectrochemical pattern formation in a self-assembled OTS monolayeron silicon, as function of the bias voltage applied between surface andtip: A) after pattern inscription, and B) after treatment of thepatterned surface with a 5 mM solution of OTS in bicyclohexyl (BCH).Non-destructive patterning gives rise to features initiallycharacterized by high friction contrast and low topography contrast (inA), the contrast being reversed (as with the 9V and 8.8V rectangles) onproceeding from A to B.

[0014]FIG. 3 shows quantitative Brewster angle FTIR spectra of: dottedline, OTS/Si monolayer self-assembled on both sides of adouble-side-polished silicon wafer substrate as that of FIG. 2; fullline, after contact for ca. 2 min (on each side) with a 3000 mesh coppergrid to which an electrical bias (13V) was applied (the siliconsubstrate being positive with respect to the grid), followed by exposureto aqueous HCl (5%) for ca. 2 hours; dashed line (1900 cm⁻¹-1300 cm⁻¹spectral region, after exposure of the acid-treated surface for ca. 0.5min to a 50 mM solution of octadecylamine in BCH. All curves representnet spectra (4 cm⁻¹ resolution) of the organic coating, aftermathematical subtraction of the spectral contributions of the bare Sisubstrate. The three-fold magnified portion of the CH₃(a) band around2964 cm⁻¹ and the full and dashed line curves in the 1900 cm⁻¹-1300 cm⁻¹spectral region were shifted vertically for clarify.

[0015]FIG. 4 shows AFM images taken during the fabrication of an arrayof silver metal islands according to route 2 in FIG. 1: (1) tip-inducedinscription of a pattern of electrooxidized OTS (OTSeo) on a base OTS/Simonolayer of the same kind as in FIGS. 2 and 3; (2) following exposureof the patterned OTS monolayer to a 5 mM solution of NTS in BCH, inorder to self-assemble a top NTS monolayer at the tip-inscribed sites;(3) after the NTS-treated sample was UV-irradiated in an atmosphere ofH₂S, in order to photochemically convert NTS to TFSM; (4) result oftreatment of the photo-reacted sample with a 2 mM aqueous solution ofHAuCl₄, followed by a silver enhancer (SE) solution, in order to depositmetal silver under the catalytic action of an ionic gold speciesadsorbed on the TFSM surface. Images (1)-(3) are friction and topographypairs simultaneously acquired in the contact mode, whereas image (4) wasobtained in the intermittent-contact mode, after unsatisfactory initialattempts of contact mode imaging.

[0016]FIG. 5 shows AFM intermittent-contact mode topographic images oftwo TFSM/OTSeo template lines produced as in FIG. 4, before (left side)and after two cycles (middle) and seven cycles (right side) of cadmiumsulfide self-assembly according to route 3. FIG. 1. Magnified top andside views of the marked square in the middle image are displayed belowit.

[0017]FIG. 6 shows AFM intermittent-contact mode topographic images ofcadmium sulfide self-assembled on a TFSM bilayer pattern (as in FIG. 5),taken after two (left side) and seven (right side) cycles of CdSself-assembly. Note the preference for edge nucleation and growth (2×CdSimage), and the accumulation of particles in the 7×CdS image compared tothe 2×CdS one.

[0018]FIG. 7 depicts a scheme of the site-defined self-assembly ofsilver metal on a thiol-top-functionalized silane monolayer (TFSM)preassembled on silicon. The silver-thiolate (—S—Ag) template surfaceobtained by the chemisorption of Ag⁺ ions on the TFSM surface (leftside) is non-destructively patterned using either a wet chemicalreduction process (lower path) or a nanoelectrochemical process (upperpath) involving the application of a DC voltage to a conducting AFM tip(see Experimental), the slightly conducting silicon substrate beingbiased negatively (reductive bias) with respect to the tip. Furtherdevelopment of the macro- and, respectively, nano-patterns of reducedsilver particles imprinted on the Ag⁺-TFSM template is shown to resultin a thicker self-assembled silver film (lower path), or self-assembledsilver islands selectively grown at tip-defined sites (upper path).

[0019]FIG. 8 illustrates a topographic contact-mode AFM images(Topometrix System) showing: (right side) portion of the edge of amillimeter-size silver electrode self-assembled on a Ag⁺-TFSM template(made from a precursor mixed monolayer with a molar ratio NTS/OTS=½) bythe wet chemical reduction and development process depicted in FIG. 7,lower path; (left side) two self-assembled silver islands grown attip-defined sites near the electrode edge shown in the image on theright, via the nanoelectrochemical reduction & development processdepicted in FIG. 7, upper path. The tip-induced reduction was done witha bias of +9.0V (applied to the same diamond-coated tip used in imaging)and a scan speed of 2 μms⁻¹. Compared to the development of theelectrode (which was done with the original silver enhancer), the silverenhancer solution used in the development of the two islands was dilutedby a factor of two and the time of contact with the surface reduced from5 min to 2 min.

[0020]FIG. 9 shows an AFM topographic record of the six successive stepsduring the fabrication of an array of nine silver islands at tip-definedsites on a Ag⁺-TFSM template as that of FIG. 8 (using the lithographicprocess depicted in FIG. 7, upper path). Images were taken in thecontact mode (Topometrix System), with the same diamond-coated tipemployed in patterning, immediately after each development step (newlyadded islands are indicated by arrows). To facilitate easy visualizationof the much smaller islands added in the last two steps, images 5 and 6are presented with expanded Z-scales. The tip-induced Ag⁺ reduction wasdone with a bias of +9.0V (on the tip) and a scan speed of 2 μms⁻¹, insteps 1 and 2, +10.0 V and 1,2 μms⁻¹ in step 3, and +10.0V and 1 μms⁻¹in steps 4-6. The concentration of the silver enhancer solution used forthe development of the last two islands was lowered by a factor of 250compared to that used for the other islands (for which the originalenhancer solution was also diluted by a factor of two) and the time ofcontact with the surface reduced from 30 s to 15 s.

[0021]FIG. 10 shows internittent contact images (NT-MDT P47 instrument)of silver nanoparticles generated by tip-induced nanoelectrochemicalpatterning (according to FIG. 7, upper path without development) of aAg⁺-TFSM template as that of FIG. 8. The two images, A and B, consist ofthree and respectively two discrete point features inscribed on the samemonolayer surface, but with two different tips. The pattern inscriptionwas done (in the contact mode) with reductive biases of 5-8V betweensurface and tip and pulse lengths of 40-150 ms applied at each inscribedpoint. Characteristic particle dimensions are evident in thedistance-height profiles (along the marked lines) shown below eachimage.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention provides organic monolayers that can bepatterned by a pattern-inscribing lithographilic instrument such as ascanning probe device, e.g. AFM, STM, or combined AFM-STM, a stampingdevice, or liquid metal, or any other device that can touch themonolayer surface and inscribe on it a chemical modification patternupon application of an electrical bias.

[0023] According to one aspect, the present invention provides apatterned organic monolayer or multilayer film self-assembled on a solidsubstrate, the pattern consisting in a site-defined surface chemicalmodification non-destructively inscribed in the organic monolayer ormultilayer by means of an electrically biased conducting scanning probedevice, stamping device and/or liquid metal or metal alloy or any otherdevice that can touch the organic monolayer or multilayer surface andinscribe therein a chemical modification pattern upon application of anelectrical bias.

[0024] The solid substrate may be silicon, e.g. a slightly conductingsilicon wafer substrate or another electrically conducting solidsubstrate

[0025] The electrically biased conducting scanning probe device may be aconducting AFM, STM or combined AFM-STM tip; the electrically biasedconducting stamping device may be a conducting metal, e.g. copper, grid,a metal plate or a stamp made of a conducting polymer or polymer-metalcomposite; and the electrically biased conducting liquid metal or metalalloy may be Hg or Ga, or liquid alloys of Hg, Ga, In, Sn or Pb.

[0026] According to one embodiment, the invention provides a patternedorganic monolayer film self-assembled on a solid substrate, wherein saidorganic monolayer is obtained by the self-assembly on the substrate of aprecursor compound, preferably a precursor compound of the formulaR′—Si—RRR, wherein each R, the same or different, is halogen, preferablychloro, lower alkyl, preferably C₁-C₆ alkyl such as methyl, hydroxy orlower alkoxy, preferably C₁-C₆ alkoxy such as methoxy, and R′ is C₁-C₃₀alkyl, partially or fully fluorinated C₁-C₃₀ alkyl, aryl such as phenyl,cycloalkyl such as cyclohexyl, polycycloalkyl such as adamantanyl,C₁-C₃₀ alkenyl, or any of these alkyl and alkenyl radicals interruptedby a cycloalkyl or aryl group, by a heteroatom selected from O, S and N,or by an ester (—O—CO—) or amide (—CO—NII—) group, the radical R′ beingsubstituted along the chain and/or terminated by a functional group.Examples of preferred C₁-C₃₀ alkyl and fluoroalkyl radicals according tothe invention are C₁₈ alkyl and C₁₀ perfluoroalkyl, respectively.

[0027] The patterned organic monolayer film according to the inventionmay be obtained, for example by the self-assembly on the substrate ofprecursor methyl-terminated silane, preferably CH₃—(CH₂)_(n)—SiCl₃wherein n=1-30. monolayers, by non-destructive patterning viatip-induced nanoelectrochemical oxidation of their top methyl groups,and optionally further derivatization of the oxidized top group. Forexample, the oxidized group may be a COOH group, that can then befurther derivatized to other desired functional groups.

[0028] In one preferred embodiment, the methyl-terminated silane isn-octadecyltrichloro-silane (CH₃—(CH₂)₁₇—SiCl₃), herein in theExperimental and Examples sections referred to as OTS.

[0029] The terminal and/or in-between functional group of the patternedorganic monolayer film may be selected from Cl, Br, OH, SH, —S—S—, CN,SCN, NH₂, (thio)carboxyl, (thio)phosphate, (thio)phosphonate,(thio)sulfate, (thio)sulfonate, (thio)carbamate, (thio)carbonate and(thio)hydroxamate groups.

[0030] In one embodiment, the functional group is carboxyl (HOOC), forexample the monolayer compound is HOOC—(CH₂)_(n)—SiRRR such asHOOC—(CH₂)₁₇—SiRRR that is obtained by oxidation of a monolayer obtainedfrom a CH₃—(CH₂)₁₇—SiCl₃ precursor. In other embodiments, the functionalgroup may be thiol (SH), disulfide (—S—S—), amino (NH₂) or phosphate(O—PO—(OH)₂) for example the monolayer compound is R′—(CH₂)_(n)—SiRRRwherein R′ is thiol, disulfide, amino or phosphate such asHS—(CH₂)₁₇—SiRRR , SiRRR—(CH₂)₁₇—S—S—(CH₂)₁₇—SiRRR, NH₂—(CH₂)₁₇—SiRRR or(OH)₂—PO—O—(CH₂)₁₇—SiRRR, that are obtained by chemical modification ofa monolayer obtained from a CH₂═CH—(CH₂)₁₇—SiCl₃ precursor.

[0031] According to another embodiment, the invention provides apatterned organic multilayer film self-assembled on a solid substrate,wherein one or more organic layers are built on top of a base monolayerobtained from a precursor compound of the formula R′—Si—RRR.

[0032] In one embodiment, the multilayer is a bilayer film, wherein onemonolayer of the formula R′—Si— is built on top of a base moiiolayerobtained from a compound of the formula R′—Si—RRR. This patternedbilayer may have, for example, one monolayer of the formulaCOOH—(CH₂)_(n)—Si—, HS—(CH₂)_(n)—Si—, H₂N—(CH₂)_(n)—Si— or(HO)₂—PO—O—(CH₂)_(n)—Si—, wherein n=1-30, preferably 17, built on top ofa monolayer obtained from a precursor of the formula R′—Si—RRR.

[0033] In another aspect, the present invention relates to a hybridinorganic-organic or organic-organic nanostructure composed of amaterial selected from a metal, a metal compound, silicon, a siliconcompound, organic metal or conducting polymer, said material beingcombined with nanoelectrochemically patterned organic monolayer ormultilayer templates self-assembled on a solid substrate, wherein saidmetal, metal compound, silicon, silicon compound, organic metal orconducting polymer is on the top and/or in-between said organicmonolayer or multilayer templates.

[0034] In this aspect of the invention, the solid substrate and thepatterned organic monolayers or multilayers are as defined above.

[0035] The metal may be a noble metal selected from Ag, Au, Pt and Ir,or a metal selected from Cu, Pb, Gi, In, Hg, Pd and Rh. The metalcompound may be selected from one or more semiconductors selected frommetal chalcogenides (i.e. sulfides, selenides, telurides), metalarsonides, and mixtures thereof; one or more metal oxides such as oxidesof iron, titanium, zinc, tin, silicon, germanium, and mixtures thereof;metal alloys, organic metals, composites of elemental metals or metalalloys with organic polymers, ceramics, and mixtures thereof.

[0036] In a further aspect, the invention provides a method for theproduction of hybrid metal-organic nanostructures as described above, bytemplate-controlled self-assembly strategy, comprising:

[0037] (i) assembling a monolayer of a silane compound terminated by afunctional group such as —SH or/and —S—S—, on a solid substrate;

[0038] (ii) binding a metal ion such as Ag to the functional group of(i); and

[0039] (ii) non-destructively patterning the top surface of said metalion-terminated layer of (ii) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern bysite-defined reduction of the metal ions to elemental metal particles.

[0040] The method can be further processed by developing the elementalmetal particles to form self-assembled metal islands or metal films

[0041] In a further embodiment, there is provided a method for theproduction of hybrid inorganic-organic or organic-organic nanostructuresof the invention, by template-controlled self-assembly strategy,comprising:

[0042] (i) assembling a monolayer of a silane compound terminated by amethyl group, on a solid substrate,

[0043] (ii) non-destructively patterning the top surface of saidmethyl-terminated layer of (i) by means of an electrically biasedconducting scanning probe device stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern byelectrochemical site-defined oxidation of the terminal methyl group, forexample to an oxygen-containing group such as COOH, and optionallyfurther chemically modifying this oxidized methyl site to anotherfunctional group; and

[0044] (iii) further generating or binding a metal, metal compound,organic metal or conducting polymer at the modified surface sites ofsaid organic layer of (ii), thus obtaining said nanostructures with acombination of a metal, metal compound, organic metal or conductingpolymer and nanoelectrochemically patterned organic monolayer templatesself-assembled on a solid substrate.

[0045] In still a further embodiment, there is provided a method for theproduction of nanostructures according to the invention, bytemplate-controlled self-assembly strategy, comprising:

[0046] (i) assembling a monolayer of a silaiie compound terminated by amethyl group, on a solid substrate:

[0047] (ii) non-destructively patterning the top surface of saidmethyl-terminated layer of (i) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern byelectrochemical site-defined oxidation of the terminal methyl group, forexample to an oxygen-containing group such as COOH, and optionallyfurther chemically modifying this oxidized methyl site to anotherfunctional group; and

[0048] (iii) binding one or more organic layers to the modified surfacesites of the organic layer obtained in (ii) above, wherein this layermay be terminated by a methyl or a vinyl group, and repeating, after thebinding of each layer, the non-destructive patterning of the top surfaceas described in (ii) above; and

[0049] (iv) further generating or binding a metal, metal compound,organic metal or conducting polymer at the modified surface sites ofsaid organic layer of (iii), thus obtaining said nanostructures with acombination of a metal, metal compound, organic metal or conductingpolymer and nanoelectrochemically patterned organic multilayer templatesself-assembled on a solid substrate.

[0050] In yet still a further embodiment, there is provided a method forthe production of nanostructures according to the invention, bytemplate-controlled self-assembly strategy, comprising:

[0051] (i) assembling a monolayer of a silane compound terminated by amethyl group, on a solid substrate;

[0052] (ii) nondestructively patterning the top surface of saidmethyl-terminated layer of (i) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern byelectrochemical site-defined oxidation of the terminal methyl group, forexample to an oxygen-containing group such as COOH, and optionallyfurther chemically modifying this oxidized methyl site to anotherfunctional group; and

[0053] (iii) binding one or more organic layers to the modified surfacesites of the organic layer obtained in (ii) above, wherein theseadditional layers are made of same or different compounds, optionallymodified by chemical processes performed after the addition of eachlayer, after the assembly of several layers, or after the assembly ofthe entire desired multilayer; and

[0054] (iv) further generating or binding a metal, metal compound,organic metal or conducting polymer on the top surface and/or in-betweensaid organic layers of (iii).

[0055] In a further aspect the invention provides a method for theproduction of a data storage medium by non-destructively inscribingchemical information on the surface of a methyl-terminated monolayer,for example as that defined above, using an electrically biasedconducting scanning probe device such as an AFM, STM or combined AFM-STMtip, or a stamping device, or a liquid metal and reading said inscribedchemical information by means of a scanning probe device such as AFM,preferably operated in the lateral force (friction) mode.

[0056] The nanostructures of the present invention have several usefulapplications. Thus, for example, in electronics, they may be used forthe fabrication of novel sub-micron and particularly nanometer-size(below˜100 nm) electronic circuits and devices, including quantumelectronic devices such as single electron switches (transistors). Inbiology, they may be used for the fabrication of patterned surfacestailored to specifically bind biological entities of interests, forbioassay purposes. In the area of miniaturized chemical or biosensors,they may be used for fabricating patterned surface structures with sitesdisplaying predesigned sensitivity toward different chemicals orbiomaterials of interest for the purpose of their identification andquantification.

[0057] Another important application relates to data storage applicationof the inscribed surface patterns, even when no nanostructure is builton the surface. Thus, the patterned organic monolayer films according tothe invention may be useful for the development of ultrahigh-densitydata storage media and surface self-assembled electronic circuits ofnanometer dimensions. The non-destructive inscription of a stablechemical pattern on the outer surface of a monolayer such as OTS orsimilar saturated silanes can be used as an attractive novel method ofultrahigh-density data storage (i.e. memory). The data can be inscribedwith an electrically biased AFM tip and read in the friction mode (alsoreferred to as lateral force microscopy-LFM). According to the presentinvention, the information can be easily inscribed in this manner on OTSor a similar monolayer, the stored information is stable and canthereafter be read in a very simple manner. There are a number ofapparent advantages this approach may offer: simple and cheap highdensity data storage medium with very high mechanical, thermal, andchemical stability, simple writing and reading. Unlike many othermethods of data storage using AFM that have been proposed, in thepresent approach the information is not stored as differences in height(topography). This will make the reading much simpler and faster. Sincethe tip is moving in contact with the surface on which the informationis stored (without damaging it), this approach will be free of theproblems encountered in present magnetic disks where the magnetic headflies above the surface and may crush into it or damage the disk becauseof the friction developing at the start of the motion.

[0058] The nanostructures according to the invention may be useful forthe production of submicron electronic circuits and devices.

[0059] The invention will now be illustrated in its various aspects bythe following non-limitative Examples and as illustrated in theaccompanying Figures.

EXAMPLES Experimental: Materials and Methods

[0060] The NTS precursor monolayers were prepared as described before(Maoz et al., 1999), on double-sided polished p-type silicon wafersubstrates (Semiconductor Processing Co., 0.5 mm thick,orientation<100>, resistivity 8-11 Ω cm). Defect-free OTS/Si monolayerswere reproducibly prepared as described before (Maoz et al., 1999), byself-assembly from a 5 mM solution of OTS in bicyclohexyl (BCH), ondouble-side-polished p-type silicon wafer substrates (SemiconductorProcessing Co., 0.5 mm thick, orientation<100>, resistivity 8-11 Ω cm).For the mixed NTS+OTS monolayers, use was made of adsorption solutionswith NTS/OTS molar ratios equal to the desired surface molar ratios. TheNTS/OTS molar ratio on the surface was found (from quantitativeFTIRspectra) to be practically identical to that of the adsorptionsolution used for the self-assembly of the respective mixed monolayer.The thiol/disulfide top functions were in sitiu introduced, through thephoto-induced radical addition of H₂S to the terminal ethylenic doublebond of NTS. Adsorption of Ag⁺ ions from a 10⁻³M aqueous solution ofsilver acetate on the TFSM surfaces, followed by rinsing with pure waterin order to remove surplus silver solution that may stick to thesurface, finally completed the preparation of the Ag⁺-TFSM templates.

[0061] Most nanoelectrochemical patterning experiments with Ag′-TFSM(FIGS. 7-10) were carried out as described before (Maoz et al., 1999)under ambient conditions (the surface being now negatively biased withrespect to the tip) with a Topometrix TMX 2010 Discoverer system usingboron-doped chemical vapor deposited (CVD) diamond-coated silicon probes(Nanosensors) with spring constants of 2-5 N/m, or silicon nitrideprobes that were coated with silver by metal evaporation. The imagesshown in FIGS. 8 and 9 were taken with the same probes (withoutelectrical bias) in the contact mode, with minimal contact forces,typically 50 nN or less, including the inherent tip-surface attraction.The writing of the patterns on the OTS/Si monolayers (FIGS. 1-6) wasdone in the contact mode, with scan speeds of 1-4 μms⁻¹, by applying apositive surface bias of 6-9V relative to the tip, depending of theambient relative humidity. Voltage regimes suitable for non-destructivepatterning were separately detenmined for each new series of experiments(see FIG. 2).

[0062] AFM images were acquired either in the contact mode (FIG. 4),with the same probe used for patterning (without electrical bias andminimal contact force, typically 50 nN or less, including the inherenttip-surface attraction), or in the intermittent-contact mode, using thesame probe (FIG. 4) or uncoated Si probes (Nanosensors) that are bettersuited for such imaging (FIGS. 5, 6).

[0063] The pattern inscription and imaging of the features shown in FIG.10 were done on a NT-MDT P47 instrument. Probes were conductiveW₂C-coated silicon tips (Silicon-MDT) with normal spring constants of0.5-2 N/m, resonance frequencies of 60-180 Khz, and Q of 80-140 Thesecharacteristics allowed using the same probe in both contact andintermittent contact modes. The former was used for pattern inscription,and the latter (without electrical bias) for imaging. Lateral smearingof the deposited metal was occasionally observed during AFM imaging inthe contact mode, because of the strong lateral forces exerted by thetip in this mode. This effect was particularly detrimental in theimaging of the small particles generated during the initialnanoelectrochemical reduction step (FIG. 7, upper path). In order toobtain satisfactory images it was necessary to switch from the contactmode used in pattern inscription to imaging in the intermittent contactmode. This could be conveniently accomplished with an NT-MDT P47instrument (see FIG. 10) purchased toward the final stages of the workdescribed herein. Pattern inscription was carried out using the systemsoftware which allows defining a dwell time and voltage bias for eachpoint of the pattern. In order to produce small feature size, thefeatures were written as single points. This could be done by holdingthe sample at ground potential and applying a positive pulse to the tip,or holding the tip at ground potential and applying a negative pulse tothe sample Except where otherwise mentioned, the entire AFM patterningand imaging work was done under normal ambient conditions (23-25° C.,50%-60% relative humidity).

[0064] For the wet chemical Ag⁺ reduction (FIGS. 7-9), drops of a 10⁻²Maqueous solution of NaBH₄ were placed on the Ag-TFSM surface for ca. 2min. then removed and the surface rinsed with drops of pure water. Thedevelopment of the silver grains generated in the initial Ag⁺ reductionstep was accomplished with a commercial silver enhancer solution (Sigma.Silver Enhancer Kit) which was further diluted with pure water whenlower metal deposition rates were desired. Drops of the enhancersolution were placed on the template surface for the specified periodsof time, then removed and the surface rinsed with drops of pure water.The removal of the drops was done by suction with a sharp pipettewithout any visible traces of liquid being left on the surface, due tothe relatively high hydrophobicity of the TFSM surface. Thus, the watercontact angles characteristic of TFSMs obtained from mixed precursormonolayers with a molar ratio NTS/OTS=I/2 vary from ca. 70°(adv.) and64° (rec.) on the silver-free surface, to ca. 67° (adv.; rec.) on thesurface fully loaded with Ag⁺ ions.

[0065] The post-patterning chemical modification and self-assemblyoperations involving liquid reagents were done with drops of the desiredsolution being placed on the patterned monolayer surface (withoutremoving the sample from the microscope stage), followed by drops of asuitable rinsing solvent. For the self-assembly of the NTS overlayer(FIGS. 1, 4-6), a drop of water was first placed on the surface for ca.2 min. then a drop of a 5 mM solution of NTS in BCH (for ca. 2 min), andfinal rinse with two drops of pure BCH followed by two drops of decalin,the NTS adsorption and the solvent rinses being repeated twice. Sincenone of the liquids employed wet the OTS monolayer, removal of the dropswas simply done by suction, without any visible traces of material beingleft on the surface.

[0066] For the conversion of NTS to TFSM, the sample was irradiated (Hglamp, λ=254 nm) for 10 min in a H₂S/Ar (1:1) atmosphere, then rinsedwith pure argon and finally twice sonicated (for ca. 15 s each) in puretoluene. Both the NTS and TFSM overlayers successfully withstand Scotchtape peeling, which was routinely applied in order to improve the AFMimages by removal of adventitious contamination from the surface. Forthe formation of CdS, the TFSM surface was loaded with Cd² ions from a 1mM solution of cadmium acetate in water (2 min adsorption, followed byrinse with two drops of pure water) and then exposed for 10 min to thesame H₂S/Ar atmosphere (without irradiation) followed by the argonrinse. The deposition of silver from the silver enhancer (SE) solutionwas done after treatment of the TFSM surface with HAuCl₄, as describedin FIG. 4.

Example 1 In-situ Surface Generation of Organic (Insulator), Metal, andSemiconductor Nanocomponents, at Predefined Surface Sites on a PatternedOTS Monolayer.

[0067]FIG. 1 depicts several different self-assembly and chemicalmodification paths suitable for in-situ surface generation of organic(insulator), metal, and semiconductor nanocomponents, at surface sitespredefined by an initial tip-inscribed oxidation pattern on OTS (OTSeo).These particular examples were selected considering the provenfeasibility of each of the separate steps (Maoz et al., 1999, 1995;Wasserman et al., 1989) making up the six routes indicated, however,many other post-patterning build-up routes are obviously conceivable,including various combinations of those shown here.

Example 1a

[0068] The modifications induced by the AFM tip in tie investigatedOTS/Si monolayers were found to depend on the applied bias and theambient relative humidity. As a rule, higher bias voltages were requiredthe lower the humidity. At constant humidity, three bias levels could bereproducibly identified giving rise to three distinct modes of patternformation. In the examples shown in FIG. 2 the features produced with+9V and +8.8V surface biases (relative to the tip) are seen to give riseto pronounced friction contrast, with only faint changes in topography(2A), the situation being reversed after exposure of the sample to asolution of OTS (2B). This strongly suggests that only the outer surfaceof the OTS monolayer is affected under these conditions, the exposure tothe OTS solution resulting in the self-assembly of a top OTS monolayerat polar surface sites produced by electrochemical oxidation of —CH₃groups under the biased tip. At a bias of ÷10V, a distinct topographicspot becomes visible (2A, right), besides the marked change in surfacefriction (2A, left) which remains partially visible after the exposureto the OTS solution (2B, left). This would suggest that excessive biasmight induce, in addition to the oxidation of the top methyls,underlayer growth of silicon oxide. with possible structural damage ofthe organic monolayer. Attempts to non-destructively pattern a silanemonolayer with a single carbon atom tail (MTS, methyltrichlorosilane)have invariably resulted in patterns showing both friction andtopography contrast, within the entire voltage range giving rise toobservable images. This is consistent with past reports of theutilization of some short tail silane monolayers as ultrathin resistsfor destructive tip-induced patterning of silicon and other materials(Sugimura et al., 1996), which would suggest that non-destructivepatterning may be possible only with compact organic films above acertain critical thickness.

[0069] Finally, at surface biases between +8.5 and +8.6V, the faintcontrast visible upon patterning only in the friction mode (2A)disappears completely from both images following the OTS treatment (2B),thus suggesting that OTS molecules from solution apparently replacedebonded monolayer molecules at the tip-affected sites rather thanself-assembling as a top monolayer. This may occur as a result oftip-induced cleavage of the siloxane bonds responsible for interlayerand layer-to-surface bonding, without oxidation of the top methylgroups. Debonding of OTS molecules may be expected to lead to higherfriction in affected monolayer regions, as the enhanced “liquid-like”character of debonded tails would give rise to an enhanced viscous dragopposing the motion of the tip.

[0070] The occurrence of such surface-solution exchange was confirmed bytreating OTS monolayers patterned in this mode with a solution of thevinyl-terminated silane NTS (see FIG. 1), instead of OTS, followed byexposure to aqueous KMnO₄. The permanganate treatment resulted in thereappearance (in the friction image only) of the tip-inscribed features,thus providing direct evidence for the oxidation of the top vinyl groupsof NTS molecules (Maoz et al., 1999) in the exchanged areas of themonolayer. Following these observations, all patterning work forconstructive nanolithography was executed under bias conditionsaffecting the top surface of the patterned monolayer only (i.e.non-destructive patterning regime), without significant underlayergrowth of silicon oxide or loss of monolayer molecules from the tip-inscribed sites (as the 9V and 8.8V features in FIG. 2). Patternsinscribed on OTS monolayers in this manner were stable for months underambient conditions while heating at 100° C. in air for ca. one hourresulted in some further enhancement of the friction contrast betweenthe affected and unaffected surface regions, without, however, causingany observable deterioration of the original pattern or appearance oftopographic modifications.

Example 1b

[0071] The nature of the non-destructive surface modification induced bythe tip was elucidated in a macro-scale simulation experiment employing,as before. (Maoz et al., 1999) a tip-mimicking copper grid (3000 mesh)pressed against the OTS/Si monolayer sample, to which a bias similar tothat used in the AFM patterning was applied. The effective contact areathus established between the metal grid and the OTS monolayer wassufficiently large to allow recording meaningful Fourier transforminfrared (FTIR) spectra of the modified surface sites. As can be seen inFIG. 3, no structural damage was caused to the monolayer uponapplication of a +13V electrical bias, the tail methylene (CH₂) bands at2917 cm⁻¹ and 2850 cm⁻¹ recorded before and after the application of theelectrical bias being superimposable. On the other hand, the methyl(CH₃) bands at 2879 cm⁻¹ and 2964 cm⁻¹ lose ca. 16% of their initialintensity (see 3×magnified inset), thus pointing to a chemicaltransformation involving the top surface of the monolayer only. Theappearance, following acidification of a C-( ) stretch band at 1713 cm⁻¹suggests oxidation of the top methyls to terminal COOH functions (Maozet al., 1995). This was unequivocally confirmed by the self-assembly, onthe oxidized OTS surface, of an ordered partial monolayer (ca. 20° offull monolayer coverage) of octadecylamine (CH₁-(CH₂)₁₇—NH₂), throughthe formation of ionic ammonium carboxylate interlayer linkages (asevidenced by the appearance of characteristic NH₃ ⁻ and COO⁻ features at1624 cm⁻¹ and 1541 cm⁻¹, 1390 cm⁻¹, respectively. (Maoz et al., 1998;Jones et al., 1987) and the concomitant disappearance of the 1713 cm⁻¹COOH band. For clarity, the octadecylamine contributions to the —CH₂—and —CH₃— bands around 2900 cm⁻¹ were omitted in FIG. 3.

[0072] The dependence of the tip-induced modification on the ambienthumidity is strongly indicative of a faradaic mechanism (Forouzan andBard. 1997), most-probably involving electrochemical generation ofreactive oxygen-rich radicals at the tip—OTS interface. The overallprocess possibly resembles the observed oxidative degradation of organicmonolayers by OH^(•) radicals electrogenerated under a scanningelectrochemical probe (Shiku and Uchida, 1997), however, the presenttip-induced oxidation is remarkable in respect of its top surfacespecificity and apparently exclusive formation of COOH functions,besides its experimental simplicity and the nanometer-scale localizationof the induced effect.

Example 1c

[0073] Once the non-destructive character and top surface specificity ofthe present patterning process were established, various routes becomeavailable, as indicated in FIG. 1, for planned post-patterning chemicalmanipulation and development. In the following, experimental evidence isprovided for the actual implementation of routes 2 and 3, thusdemonstrating the successful integration of wet chemical, gas phase,photochemical and catalytic processes, together with organic andinorganic self-assembly, in multistep sequences of nanofabrication onhighly stable monolayer and bilayer templates produced by the presentapproach.

[0074] For example, FIG. 4 shows AFM “snapshots” taken after each of thefour consecutive steps involved in the fabrication of an array of silvermetal islands according to route 2 in FIG. 1. The triangular array ofelectrooxidized OTS (OTSeo) rectangles (ca. 400 nm×500 nm each)inscribed with the biased tip is clearly seen in the friction image (1,left), the corresponding topographic image (1, right) showing only avery small height increase, of 2-3 Å, possibly arising from theformation of the top COOH groups. Following the self-assembly of an NTSoverlayer (2), the expected contrast reversal between friction andtopography is observed, the NTS/OTSeo bilayer pattern showing up as aca. 2.2 nm height increase in the topographic image (2, right) only. Theconversion of NTS to the polar TFSM derivative, upon the photochemicaladdition of H₂S to the terminal double bond of NTS, is marked by thereappearance (with enhanced contrast) of the tip-inscribed pattern inthe friction image (3, left), as well as by a 3-5 Å further heightincrease in the corresponding topographic image (3, right), associatedwith the addition of the bulky sulfur functions. Finally, selectivesilver metal self-assembly on the TFSM template was achieved (4) byfirst forming a gold-TFSM complex, which subsequently catalyzes thedeposition of the metal from a silver enhancer (SE) solution.

[0075] Reproducible topographic images of the metal islands wereobtained in the intermittent-contact mode (4), following initialattempts of contact mode imaging which suffered from lateral smearing asa result of the relatively high shear forces exerted by the tip. Exceptfor the damage caused in the contact mode imaging, the silver islandsproduced (ca. 50 nm height) are seen to faithfully follow the triangulararray of rectangles initially inscribed with the tip. This confirms thesuccessful implementation (according to route 2 of FIG. 1) of threeconsecutive post-patterning chemical manipulation steps, involving thesite-selective self-assembly of an organic overlayer, the selectivephotochemical modification of the overlayer outer surface, and the finalutilization of the modified surface as template for the control of thesurface self-assembly of a metal, the entire multistep process beingcarried out with very good structural conservation of both the modifiedand unmodified portions of the patterned OTS monolayer as well as of theorganic overlayer self-assembled at the tip-inscribed sites.

Example 1d

[0076] In FIGS. 5-6 we show examples of particulate cadmium sulfidefeatures produced according to route 3 in FIG. 1. Unlike the depositionof silver from the silver enhancer solution, the total amount of CdSgenerated in this process is controlled by the finite Cd^(2′) bindingcapacity of the TFSM template. Upon exposure of the Cd^(2.)-TFSM to H₂S,the free TFSM template is regenerated and so can be reused as a Cd^(2.)binder. Thus it becomes possible to repeatedly deposit CdS, in a cyclic.template-controlled process consisting of Cd^(2.) adsorption on the TFSMsurface, followed by exposure to H₂S. FIG. 5 shows two TFSM/OTSeotemplate lines (ca. 2,400 nm×60-100 nm each) before (top, left) andafter two and seven such cycles of cadmium sulfide self-assembly (2×CdSand 7×CdS, respectively). Accumulation and growth of CdS particles onthe TFSM surface is evident from a comparison of the 2×CdS and 7×CdSimages, the latter displaying aggregates of coalesced particles alignedalong the TFSM template lines. An inspection of the magnified top andside views (bottom images) of the marked line segment in the 2×CdS imagereveals discrete particles with heights below 3 nm and lateraldimensions below 30 nm, i.e. significantly smaller than the width (ca.90 nm) of the template line itself. This is a desirable consequence ofthe self-assembly of a 3D object on a laterally confined 2D templatethat can supply only a limited amount of template-bound precursorspecies (Cd²⁺ ions) during the self-assembly process. Thus, properlydesigned monolayer templates could be used to control the dimensions ofself-assembled semiconductor and metal surface features below the actualsize of the writing tip.

[0077] Other special self-assembly effects arising from such lateralconfinement can be expected, as suggested by the peculiar formation ofcadmium sulfide “bowls” (FIG. 6) due to preferential nucleation andgrowth of CdS particles at the periphery of TFSM/OTSeo template domainswith certain characteristic lateral dimensions. The good structuralpreservation of both the OTS base monolayer and the top-assembledorganic template is evident here. like in FIGS. 4-5, from the highfidelity of the step-to-step pattern transfer, no structural defectsbeing identified in images of the same surface region recorded atdifferent consecutive stages during the fabrication process.

[0078] In conclusion, while the mechanism of the tip-inducedelectrooxidation of terminal methyls in monolayer systems of the kindpresently studied remains to be elucidated, the results obtainedconvincingly demonstrate the successful utilization of such “inert”monolayers as extremely versatile base templates in constructivenanolithography. This paves the way for a more thorough investigation ofthe synthetic potential of this promising new nanofabrication approach,toward its eventual application in the development of novelultrahigh-density data storage media and surface self-assembledelectronic circuits of nanometer dimensions.

Example 2 Site-defined Silver Metal Self-assembly on a PatternedMonolayer Template

[0079] Starting with a thiol-top-functionalized silane monolayer (TFSM)with silver ions chiemisorbed on its outer surface (Ag⁺-TFSM) metallicsilver nanoparticles are generated at selected surface sites by eitherwet chemical or tip-induced electrochemical reduction of thesurface-bound metal ions. As illustrated in FIG. 7, the conventional wetchemical reduction (e.g. with aqueous NaBH₄) can be used to covermacroscopic surface areas, with lateral dimensions between ca. 0.5millimeter (micropipette delivery may allow targeting of the reducingsolution to sub-micron surface areas) to several centimeters, whereassite-defined reduction of the silver thiolate in the micron down to thenanometer-size range can be achieved with the help of a conducting AFMtip.

[0080] If desired, larger metal islands and thicker films, useful aselectrical contacts and current leads, may be grown by further chemicaldevelopment of the initially generated silver particles (FIG. 7). Thus,silver metal structures are assembled according to a predefined design,by non-destructively imprinting chemical information on the outersurface of a stable, solid supported organic monolayer that performs thefunction of an active template for spatial control of the metalself-assembly. As in the site-defined bilayer self-assembly demonstratedbefore (Maoz et al., 1999), once an initial pattern is inscribed on theouter surface of the base monolayer template, all subsequent operationsleading to the final desired surface structure consist of in situchemical modifications and self-assembly processes only, the monolayertemplate being preserved as an integral part of the resulting finalstructure. Thus, unlike lithographic methods based on resist removal andetch pattern-transfer technologies (See, for example, Mino et al., 1994;Lercel et al., 1996; Thywissen et al., 1997; Xia et al., 1998; Perkinset al., 1994; Schoer et al., 1994; Sugimura et al., 1996; Tully et al.,1999) constructive nanolithography takes advantage of surfaceself-assembly processes that, by their very nature, hold promise forattractive new developments in nanofabrication, both in terms of thecombined chemical-architectural diversity they offer and ultimateachievable miniaturization, beyond the inherent limits of the primarypattern inscription step. For example, processes of spatially-confinedself-assembly can be utilized to generate monolayer-bound metalparticles (at tip-inscribed sites) significantly smaller than theeffective size of the AFM tip used for patterning, as shown below.

Example 2a

[0081] Densely packed, defect-free TFSMs with variable surfaceconcentrations of sulfur were produced photochemically from high qualityNTS precursor monolayers (NTS, 18-nonadecenyltrichlorosilane,CH₂═CH—(CH₂)₁₇—SiCl₃) (Maoz et al., 1999) and NTS+OTS mixed monolayers(OTS, n-octadecyltrichlorosilane, CH₃—(CH₂)₁₇—SiCl₃) self-assembled onslightly conducting silicon wafer substrates (see Experimental). Most ofthe present work was done on mixed monolayers with a molar ratioNTS/OTS=½, which combine two desirable properties; a sufficiently largesurface density of the sulfur-containing functions (generated from theterminal vinyl groups of NTS), together with enhanced surfacehydrophobicity (due to the large percentage of outer —CH₃ groupscontributed by OTS). The relatively high hydrophobicity of suchmonolayers permits easy handling of the wet chemical surface treatmentsand facilitates lateral confinement of the chemical reduction process,by the use of well defined non-spreading droplets of the liquidreagents. In situ top-functionalized monolayers obtained by this methodusually expose both thiol and disulfide surface functions, the formationof the latter depending on the packing density of the top vinyl groupsin the precursor monolayer. Considering the comparable silver-bindinigperformance of the thiol and the disulfide, no attempt was made todeliberately control the exact content of these functions in thedifferent sulfur-containing monolayers examined during this experiment.Disulfide surface functions obtained as the primary main product of thephoto-induced reaction of H₂S with pure NTS monolayers (seeExperimental) may subsequently be converted to thiol groups by chemicalreduction with a suitable reducimg reagent.

[0082] For brevity, we use here the tern TFSM in a general sense,although the actual percentage of thiol groups in different TFSMs maythus vary, depending on the NTS/OTS molar ratios of the respectiveprecursor monolayers.

[0083] The formation of nanoparticles of metallic silver+free thiolgroups upon the wet chemical reduction of silver-thiolate monolayersurface groups (carried out over macroscopic surface areas; FIG. 7,lower path) was confirmed by UV-vis spectroscopy, X-ray photoelectronspectroscopy (XPS) and AFM imaging, while the structural stability ofthe template monolayers was routinely checked by taking quantitativeFourier transform infrared (FTIR) spectra of the investigated samplesbefore and after each of the chemical operations indicated in FIG. 7.Silver nanoparticles generated by such wet chemical reduction ofAg⁺-TFSM surfaces could be further developed (FIG. 7) using a silverself-assembly process that takes place exclusively around preformedsilver metal nuclei, while being practically inactive at surface sitesexposing unreduced silver ions only. In this manner, once a pattern ofreduced silver is generated, further chemical deposition of silver metalwould selectively amplify it thus resulting in effective development ofthe initially inscribed chemical information (FIG. 7). This was realizedby the application of a metal-catalyzed silver enhancer solution whichdeposits silver only on metal-seeded sites (Braun et al., 1998). Therate of deposition and total amount of deposited silver were controlledby a number of adjustable parameters, such as the concentration of theenhancer solution and the time of contact with the activated surface(see FIGS. 8, 9). The selective development of silver metal grains upontreatment with the enhancer solution could thus be used as a sensitiveindicator of the presence and location of reduced silver on the treatedsurface. This property was fully exploited in the nanoelectrochemicalpatterning experiments described in the following, for an unequivocalidentification of tip-geenerated silver particles and theirdifferentiation from grainy features originating in adventitious surfacecontamination that may also show up in the AFM images.

Example 2b

[0084] Using wet chemical reduction (FIG. 7, lower path),millimeter-size conducting silver electrodes could be produced withinminutes on Ag⁺-TFSM surfaces by a very simple procedure consisting ofsequential placement and removal (with a pipette) of small drops of thereducing solution, pure water, the silver enhancer solution, and againpure water (see Experimental). The silver metal deposition was found tobe well defined by the position and size of each reducing drop, no metalfilm formation being observed outside the circumference of thecorresponding reduced surface spots (see below). The chemical andnanoelectrochemical processes (FIG. 7, lower and upper paths,respectively) can be easily combined, which may be particularly usefulfor the fabrication of electrical contacts between the macroscopic worldand a self-assembled nanocircuit. This is demonstrated, for example, bythe successful tip-induced generation of two silver islands atpreselected surface sites near the edge of a conducting silver electrodefabricated by the wet chemical procedure described above (FIG. 8). It isof interest to note in FIG. 8 the sharp edge of the electrode and thefact that silver metal was selectively deposited only within those areasof the Ag⁺-TFSM template that were either exposed to the chemicalreducing reagent or scanned with the tip under appropriate reductivebias prior to the application of the enhancer solution, despite thepresence of Ag⁺ ions on the entire imaged surface. This points to theequivalence of the chemical and tip-induced processes, the formation ofreduced silver grains being effectively confined to those surface sitesdeliberately marked with the biased tip during the initial patterningstep.

Example 2c

[0085] For many applications, it would be advantageous to be able tosequentially add new elements to a growing nanostructure, whilecontinuously monitoring the entire build-up process with the help of anon-destructive inspection tool. Constructive nanolithography offersthis option, as illustrated by the site-defined self-assembly, in sixseparate steps, of an array of nine silver islands (FIG. 9), using thenanoelectrochemical reduction and development process depicted in FIG.7. In the example given in FIG. 9, individual islands as well as a pairof islands (step 4) were added sequentially to an initial set of threeislands, the resulting structure being in situ imaged (with the sameconductive diamond tip used for patterning) before and after each of theoperations involved in its construction. To demonstrate the flexibilityof this self-assembly approach, the last added two islands (steps 5 and6) were intentionally made much smaller than the first seven, withheights below 50 nm and lateral dimensions below ca. 0.4 μm. Thesuccessful implementation of such a sequence of site-defined metaldeposition steps is obviously a consequence of the fact that no silvermetal is deposited in the absence of intentional site activation by thetip, again pointing to the formation of metallic grains upon thereduction of surface-bound silver ions under the tip.

Example 2d

[0086] Examples of AFM images of primary (undeveloped) metal particles,generated by tip-induced nanoelectrochemical reduction of TFSM-boundsilver ions, are given in FIGS. 10A-10B. FIG. 10A shows clusters ofnanoparticles with typical heights of 2-3 nm and lateral dimensions ofthe individual particles between 20-30 nm, while in FIG. 10B one can seeisolated particles with heights of 5-6 nm and lateral dimensions of ca.30 nm. Since these point features, in 10A and 10B, were produced on thesame monolayer surface, but with different tips (also used for imaging),the resulting particles most probably reflect the interplay between tipsize and shape because of the convolution with the tip, the measuredlateral dimensions of the particles may be overestimated) and thenucleation and growth kinetics of metal crystallites following thereduction of silver ions present within surface domains affected by thetip. It is thus interesting to note the formation of clustered metalparticles, each particle having lateral dimensions 4-5 times smallerthan those of the cluster itself (e.g. #3 in image 10A), i.e.significantly smaller than the overall size of the tip-affected domain.

Example 2e

[0087] By analogy with the wet chemical reduction process, the selectivedeposition of silver metal from the silver enhancer solution at thetip-inscribed surface sites strongly suggests that, under the conditionsof these experiments, the tip-induced transformation indeed involveslocal electrochemical reduction of the surface-bound Ag⁺ ions toelemental silver. This view is confirmed by the results of a series ofadditional experiments, briefly summarized in the following, whichprovide further insight into the mechanism of the tip-induced Ag⁺reduction as well as into other possible modes of constructivenanolithography:

[0088] Upon treatment with the silver enhancer solution, no developmentwas observed after the Ag⁺-TFSM surface was scanned with a conductingdiamond tip under reverse bias (i.e. tip negative, Si substratepositive). Likewise, a pattern “written” with a positively biased tipand then “rewritten” with the same tip negatively biased could not bedeveloped. This implies that elemental silver generated in the reductivescanning mode undergoes oxidation when the same scan is repeated in theoxidative mode. Scanning again the same area in the reductive mode (tippositive) enabled development, which implies reversibility of theoxidation process.

[0089] No development was found to occur when Ag⁺-TFSM surfaces werescanned in an atmosphere of dry nitrogen, irrespective of the biasapplied to the diamond tip. This clearly indicates that the tip-inducedreduction of Ag⁺-TFSMs, like the previously reported tip-inducedoxidation of NTS monolayers (maoz et al., 1999), is a water-mediatedfaradaic process (Forouzan and Bard, 1997) in which atmospheric watervapor condensing at the tip (Piner and Mirkin, 1997) plays an essentialrole (Sugimura and Nakagiri, 1997). The formation of elemental silverconceivably involves electrochemical reduction at the Ag⁺-TFSM surface(cathode) and oxidation of water at the tip (anode);

[0090] Surface (negative): 4R—S—Ag⁺+4e⁻+4H₂O→4R—SH+4Ag⁰+4OH⁻

[0091] Tip (positive): 2H₂O→O₂+4H⁺+4e⁻

[0092] Overall process: 4R—S⁻Ag⁺+2H₂O→4R—SH+4Ag⁰+O₂

[0093] No deposition of silver from the silver enhancer solution wasobserved on a monolayer of OTS/Si or on bare silicon (after scanningwith the diamond tip in the reductive mode), which confirms that nospurious surface processes, unrelated to the template-bound Ag⁺ ions,could be responsible for the observed effects.

[0094] Finally, rather intriguing results were obtained when, in anattempt to locally deliver Ag⁺ ions to a silver-free TFSM surface, thediamond-coated silicon tip was replaced with a silver-coated siliconnitride tip. In air, the reductive scanning mode (tip positive) produceda pattern which could not, however, be developed with the silverenhancer solution. On the other hand, development occurred when thereductive scannings was done under dry nitrogen. No effect was observed,either in air or in the dry nitrogen atmosphere, when the TFSM wasscanned under reverse bias (tip negative) or without electrical bias.These observations can be rationalized if we assume that in humid airthe positively biased silver tip (anode) is oxidized (Forouzan and Bard,1997) and silver oxide or hydroxide particles rather than Ag⁺ ions arereleased to the TFSM surface (cathode), where water is reduced;

[0095] Tip (positive): 2Ag⁰+H₂O→Ag₂O+2H⁺+2e⁻

[0096] Surface (negative): 2H₂O+2e^(−→H) ₂+2OH⁻

[0097] Overall process: 2Ag⁰+H₂O→Ag₂O+H₂

[0098] In dry nitrogen, a water-free electrochemical process occurs,whereby Ag⁺ ions generated at the positive silver tip adsorb on thenegative TFSM surface, where further rapid reduction to elemental silveroccurs;

[0099] Tip (positive): Ag⁰→Ag⁺+e⁻

[0100] Surface (negative): R—SH+Ag⁺+e⁻→R—SH+Ag⁰

[0101] Overall process: Transfer of Ag⁰ from tip to surface.

[0102] Taken together, the combined results of the present described“macro-” and “micro-size” experiments provide conclusive evidence forthe feasibility of site-defined self-assembly of silver metal on bothchemically and nanoelectrochemically patterned monolayer templates withsulfur-containing outer groups. Two modes of non-destructive patterningwith electrically biased AFM tips were examined, both of which generatean initial, template-stabilized pattern of elemental silver grains,which can be further developed by treatment of the surface with a silverenhancer solution. In one mode, Ag⁺ ions bound to the surface of thetemplate are locally reduced with a silver-free conductive tip operatingin normal ambient conditions, whereas in the second mode, elementalsilver is locally transferred from a silver-coated tip to a silver-freetemplate surface scanned under dry nitrogen. The available evidencesuggests that a water-mediated faradaic mechanism is, most probably,operative in the first mode, whereas the second mode involves a dryelectrochemical process facilitated by the direct contact betweenoppositely biased tip and template. The net result of the latter processmay thus be regarded as representing electrically-driven transport ofsilver metal from the tip to a stable, silver-binding monolayer surface,the tip acting here as a nanometric solid-state “fountain-pen” thatdelivers a solid “ink” (silver metal) to a “paper” consisting of afunctional surface with chemical affinity for this particular “ink”. Ananalogous “inverted” process, referred to as “dip-pen” nanolithography,was recently reported, whereby a molecular “ink” made ofmonolayer-forming thiol molecules is mechanically delivered from a“ink-loaded” AFM tip acting as “pen” to a solid gold surface acting as“paper” (Piner et al., 1999).

[0103] In conclusion, proof-of-concept experiments have been carried outdemonstrating the possible utilization of constructive nanolithographyas a versatile approach to the in situ chemical fabrication of spatiallydefined metal stuctures on organic monolayer templates. It was furthershown that a simple change of experimental conditions, involving the tipmaterial, the composition of the template surface, and the compositionof the ambient atmosphere, may result in a different, yet useful mode ofnanoelectrochemical surface patterning which points to the versatilityand wide applicability of the method.

[0104] While the present proof-of-concept study was not intended toexplore the limits of miniaturization achievable by the described newapproach, we should emphasize that in principle, it offers attractiveoptions for miniaturization beyond the smallest surface features thatmight be directly generated through the patterning process (FIG. 10A).This follows from the fact that a 3D object, such as a metal orsemiconductor particle, grown from a sub-monolayer supply ofsurface-bound metal ions (available within a lithographically defined 2Dmonolayer domain with limited ion binding capacity), must necessarily besmaller than the domain itself. Relying on such processes of “laterallyconfined” self assembly and growth rather than on etch and removal ofmaterial, constructive naiolittiography is (unlike most otherlithographic schemes) intrinsically adapted to transcend the limits ofminiaturization inherent in the primary patterning process itself. Thisaspect holds great promise for a series of applications, particularly innanoelectronics (Ahmed, 1997) that are critically dependent on theability to assemble and address complex functional structures withprecisely defined nanometric dimensions.

[0105] References

[0106] 1 H. Ahmed, J. Vac. Sci. Technol. B 1997, 15, 2101.

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1. A patterned organic monolayer or multilayer film self-assembled on asolid substrate, the pattern consisting in a site-defined surfacechemical modification non-destructively inscribed in the organicmonolayer or mnultilayer by means of an electrically biased conductingscanning probe device, stamping device and/or liquid metal or metalalloy or any other device that can touch the organic monolayer ormultilayer surface and inscribe therein a chemical modification patternupon application of an electrical bias.
 2. The patterned organicmonolayer or multilayer film according to claim 1, wherein the solidsubstrate is silicon or another electrically conducting solid substrate.3. The patterned organic monolayer or multilayer film according to claim1 or 2, wherein said electrically biased conducting scanning probedevice is a conducting AFM, STM or combined AFM-STM tip.
 4. Thepatterned organic monolayer or multilayer film according to claim 1 or2, wherein said electrically biased conducting stamping device is aconducting, metal grid, a metal plate or a stamp made of a conductingpolymer or polymer-metal composite.
 5. The patterned organic monolayeror multilayer film according to claim 1 or 2, wherein said electricallybiased conducting liquid metal or metal alloy is selected from Hg, Ga,and alloys of Hg, Ga, In, Sn or Pb).
 6. A patterned organic monolayerfilm according to any one of claims 1 to 5, wherein said organicmonolayer is obtained by the self-assembly on the substrate of aprecursor compound of the fonnula R′—Si—RRR, wherein each R, the same ordifferent, is halogen, lower alkyl, hydroxy or lower alkoxy, and R′ isC₁-C₃₀ alkyl, partially or fully fluorinated C₁-C₃₀ alkyl, aryl,cycloalkyl, polycycloalkyl, C₁-C₃₀ alkenyl, or any of such alkyl andalkenyl radicals interrupted by a cycloalkyl or aryl group or by aheteroatom selected from O, S and N, or by an ester (—O—CO—) or amide(—CO—NH—) group, the radical R′ beiing, substituted along the chainand/or terminated by a functional group.
 7. The patterned organicmonolayer film according to claim 6, wherein said organic monolayer isobtained from a precursor methyl-terminated silane, preferablyCH₃—(CH₂)_(n)—S Cl₃ wherein n=1-30, by non-destructive patterning viatip-induced nanoelectrochemical oxidation of the top methyl groups, andoptionally further derivatization of the oxidized top groups.
 8. Thepatterned organic monolayer film according to claim 6, wherein n is 17and said methyl-terminated silane is n-octadecyltrichlorosilane(CH₃—(CH₂)₁₇—SiCl₃).
 9. The patterned organic monolayer film accordingto any one of claims 6 to 8, wherein said terminal or in-betweenfunctional group is selected from Cl, Br, OH, SH, —S—S—, CN, SCN, NH₂,(thio)carboxyl, (thio)phosphate, (thio)phosphonate, (thio)sulfate,(thio)sulfonate, (thio)carbamate, (thio)carbonate and (thio)hydroxamate.
 10. The patterned organic monolayer film according to claim9, wherein said functional group is COOH.
 11. The patterned organicmonolayer film according to claim 10, wherein the monolayer compound isHOOC—(CH₂)₁₇—SiR₃ that is obtained by oxidation of a monolayer obtainedfrom a CH₃—(CH₂)₁₇—SiCl₃ precursor.
 12. The patterned organic monolayerfilm according to claim 9, wherein said functional group is HS—, —S—S—,NH₂, or (OH)₂—PO— O—.
 13. The patterned organic monolayer film accordingto claim 12, wherein said thiol, disulfide, amino or phosphatefunctionalized monolayer is obtained by chemical modification of amonolayer obtained from a CH₂═CH—(CH₂)₁₇—SiCl₃ precursor.
 14. Apatterned organic multilayer film according to any one of claims 1 to 5,wherein one or more organic layers are built on top of a base monolayerobtained from a precursor compound of the formula R′—Si—RRR, wherein Rand R′ are defined as in claim 6 above.
 15. A patterned organic bilayerfilm according to claim 14, wherein one monolayer of the formula R′—Si—is built on top of a base monolayer obtained from said precursorcompound of the formula R′—Si—RRR.
 16. The patterned organic bilayerfilm according to claim 15, wherein one monolayer of the formulaCOOH—(CH₂)_(n)—Si—, HS—(CH₂)_(n)—Si—, H₂N—(CH₂)_(n)—Si— or(HO)₂—PO—O—(CH₂)_(n)—Si—, wherein n=1-30, preferably 17, is built on topof a monolayer obtained from said precursor compound of the formulaR′—Si—RRR.
 17. The patterned organic bilayer film according to claim 16,wherein said top monolayers of the formulas COOH—(CH₂)_(n)—Si—,HS—(CH₂)_(n)—Si—, H₂N—(CH₂)_(n)—Si— or (HO)₂—PO—O—(CH₂)_(n)—Si—, areobtained by the chemical modification of a monolayer obtained from avinyl-terminated silane precursor of the formula CH₂═CH—(CH₂)_(n)—SiCl₃,wherein n=1-30, preferably
 17. 18. A hybrid inorganic-organic ororganic-organic nanostructure composed of a material selected from ametal, a metal compound, silicon, a silicon compound, organic metal orconducting polymer, said material being combined withnanoelectrochemically patterned organic monolayer or multilayertemplates self-assembled on a solid substrate, wherein said metal, metalcompound, silicon or silicon compound, organic metal or conductingpolymer, is on the top and/or in-between said organic monolayer ormultilayer templates.
 19. A nanostructure according to claim 18, whereinthe solid substrate is silicon or another electrically conducting solidsubstrate.
 20. A nanostructure according to claim 18 or 19, wherein saidpatterned organic monolayers or multilayers are as defined in any one ofclaims 1 to
 17. 21. A nanostructure according to any one of claims 18 to20, wherein said metal is a noble metal selected from Ag, Au, Pt, andIr, or a metal selected from Cu, Pb, Ga, In, Hg, Pd, and Rh.
 22. Ananostructure according to any one of claims 18 to 20, wherein saidmetal compound is selected from one or more semiconductors selected frommetal chalcogenides, metal arsenides, and mixtures thereof; one or moremetal oxides (selected from oxides of iron, titanium, zinc, tin,silicon, germanium, and mixtures thereof); metal alloys, organic metals,conducting polymers, composites of elemental metals or metal alloys withmetal compounds, organic polymers, ceramics, and mixtures thereof.
 23. Amethod for the production of hybrid metal-organic nanostructuresaccording to claim 18, by template-controlled self-assembly strategy,comprising: (i) assembling a monolayer of a silane compound terminatedby a functional group such as —SH and/or —S—S— on a solid substrate;(ii) binding a metal ion such as Ag⁺ to the functional group of (i); and(iii) non-destructively patterning the top surface of said metalion-terminated layer of (ii) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern bysite-defined reduction of the metal ions to elemental metal particles.24. The method according to claim 23, which comprises further developingthe elemental metal particles to form self-assembled metal islands ormetal films.
 25. A method for the production of hybrid inorganic-organicor organic-organic nanostructures according to claim 18, bytemplate-controlled self-assembly strategy, comprising: (i) assembling amonolayer of a silane compound terminated by a methyl group, on a solidsubstrate; (ii) non-destructively patterning the top surface of saidmethyl-terminated layer of (i) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern byelectrochemical site-defined oxidation of the terminal methyl group, forexample to an oxygen-containing group such as COOH, and optionallyfurther chemically modifying this oxidized methyl site to anotherfunctional group; and (iii) further generating or binding a metal, metalcompound, organic metal or conducting polymer at the modified surfacesites of said organic layer of (ii), thus obtaining said nanostructureswith a combination of a metal, metal compound, organic metal orconducting polymer and nanoelectrochemically patterned organic monolayertemplates self-assembled on a solid substrate.
 26. A method for theproduction of nanostructures according to claim 18, bytemplate-controlled self-assembly strategy, comprising: (i) assembling amonolayer of a silane compound terminated by a methyl group, on a solidsubstrate; (ii) non-destructively patterning the top surface of saidmethyl-terminated layer of (i) by means of an electrically biasedconducting scanning probe device, stamping device and/or liquid metal ormetal alloy or any other device that can touch said organic monolayersurface and inscribe therein a chemical modification pattern uponapplication of an electrical bias, thus forming the pattern byelectrochemical site-defined oxidation of the terminal methyl group, forexample to an oxygen-containing group such as COOH, and optionallyfurther chemically modifying this oxidized methyl site to anotherfunctional group; and (iii) binding one or more organic layers to themodified surface sites of the organic layer obtained in (ii) above,wherein this layer may be terminated by a methyl or a vinyl group, andrepeating after the binding of each layer, the non-destructivepatterning of the top surface as described in (ii) above; and (iv)further generating or binding a metal, metal compound, organic metal orconducting polymer at the modified surface sites of said organic layerof (iii), thus obtaining said nanostructures with a combination of ametal, metal compound, organic metal or conducting polymer andnanoelectrochemically patterned organic multilayer templatesself-assembled on a solid substrate.
 27. A method for the production ofnanosturctures according to claim 18, by template-controlledself-assembly strategy, comprising: (i) assembling a monolayer of asilane compound terminated by a methyl group, on a solid substrate; (ii)non-destructively patterning the top surface of said methyl-terminatedlayer of (i) by means of an electrically biased conducting scanningprobe device stamping device and/or liquid metal or metal alloy or anyother device that can touch said organic monolayer surface and inscribetherein a chemical modification pattern upon application of anelectrical bias, thus forming the pattern by electrochemicalsite-defined oxidation of the terminal methyl group, for example to anoxygen-containing group such as COOH, and optionally further chemicallymodifying this oxidized methy site to another functional group; and(iii) binding one or more organic layers to the modified surface sitesof the organic layer obtained in (ii) above, wherein these additionallayers are made of same or different compounds, optionally modified bychemical processes performed after the addition of each layer, after theassembly of several layers, or after the assembly of the entire desiredmultilayer; and (iv) further generatinig or binding a metal, metalcompound, organic metal or conducting polymer on the top surface and/orin-between said organic layers of (iii).
 28. A method for the productionof a data storage medium by non-destructively inscribing chemicalinformation on the surface of a methyl-terminated monolayer, for exampleas that defined in claim 8 above using an electrically biased conductingscanning probe device such as an AFM, STM or combined AFM-STM tip, or astamping device, or a liquid metal and reading said inscribed chemicalinformation by means of a scanning probe device such as AFM, preferablyoperated in the lateral force (friction) mode.
 29. Use of patternedorganic monolayer or multilayer films according to claim 1, for thedevelopment of ultrahigh-density data storage media and surfaceself-assembled electronic circuits of nanometer dimensions.
 30. Use ofnanostructures according to claim 18, for the production of submicronelectronic circuits and devices.